1. Field of the Invention
The present invention relates to methods for increasing hydrolysis of a cellulosic material with an enzyme composition in the presence of a cellobiose dehydrogenase.
2. Description of the Related Art
Cellulose is a polymer of the simple sugar glucose linked by beta-1,4-bonds. Many microorganisms produce enzymes that hydrolyze beta-linked glucans. These enzymes include endoglucanases, cellobiohydrolases, and beta-glucosidases. Endoglucanases digest the cellulose polymer at random locations, opening it to attack by cellobiohydrolases. Cellobiohydrolases sequentially release molecules of cellobiose from the ends of the cellulose polymer. Cellobiose is a water-soluble beta-1,4-linked dimer of glucose. Beta-glucosidases hydrolyze cellobiose to glucose.
Cellobiose dehydrogenases are secreted as a component of the cellulose-degrading proteome of various fungal species. The enzymes are single subunit, multi-domain enzymes catalyzing the oxidation of cellobiose to cellobionolactone, with concomitant reduction of a variety of substrates. The oxidizing substrates depend largely on the specific cellobiose dehydrogenase and include, but are not limited to, iron, oxidized phenolics, cytochrome C, metal ions, and molecular oxygen.
Several biological functions have been suggested or inferred for cellobiose dehydrogenase activity. These include, but are not limited to, hydroxide radical-mediated cellulose cleavage, delignification, wood invasion, pathogen defense, and selective advantage in mixed fungal population.
Cellobiose dehydrogenase has been shown to contribute to the depolymerization of cellulose, which has been ascribed to generation of reactive oxygen species (ROS) by cellobiose dehydrogenase (Mansfield et al., 1997, Appl. Environ. Microbiol. 63(10): 3804-3809) in the presence of cellobiose and Fe(II) or other reductive metals. Cellobiose dehydrogenase has also been attributed a delignification function, again in the generation of ROS in conjunction with laccase or lignin peroxidase. Cellobiose dehydrogenase generation of ROS has also been suggested as a possible defense mechanism against pathogens and against more aggressive fungal species that utilize lignocellulose.
While the art suggests that cellobiose dehydrogenase is depolymerizing and thus mildly enhancing when added to cellulase mixtures (Mansfield et al., 1997, supra), the present invention observes that cellobiose dehydrogenase activity can be inhibitory to cellulase compositions.
The present invention provides methods for reducing the inhibitory effect of a cellobiose dehydrogenase on hydrolysis of cellulosic materials by enzyme compositions.